Fixed: Fix app crash if failed to start TermuxService while in background due to android bg restrictions

The crash happens due to android 8.0 background restrictions if TermuxActivity is not in foreground/whitelist and attempts to start TermuxService. With this commit, the app will not crash but will just exit with a toast message.

https://developer.android.com/about/versions/oreo/background#services

https://cs.android.com/android/platform/superproject/+/android-12.0.0_r4:frameworks/base/services/core/java/com/android/server/am/ActiveServices.java;l=722

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.termux/com.termux.app.TermuxActivity}: java.lang.IllegalStateException: Not allowed to start service Intent { cmp=com.termux/.app.TermuxService }: app is in background uid UidRecord{533ae62 u0a187 TPSL idle procs:1 seq(0,0,0)}
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3082)
 at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
 at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
 at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1832)
 at android.os.Handler.dispatchMessage(Handler.java:106)
 at android.os.Looper.loop(Looper.java:201)
 at android.app.ActivityThread.main(ActivityThread.java:6821)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
Caused by: java.lang.IllegalStateException: Not allowed to start service Intent { cmp=com.termux/.app.TermuxService }: app is in background uid UidRecord{533ae62 u0a187 TPSL idle procs:1 seq(0,0,0)}
 at android.app.ContextImpl.startServiceCommon(ContextImpl.java:1587)
 at android.app.ContextImpl.startService(ContextImpl.java:1542)
 at android.content.ContextWrapper.startService(ContextWrapper.java:674)
 at com.termux.app.TermuxActivity.onCreate(TermuxActivity.java:242)
 at android.app.Activity.performCreate(Activity.java:7224)
 at android.app.Activity.performCreate(Activity.java:7213)
 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2927)
 ... 11 more

Closes #2552

app/src/main/res/values/strings.xml

@@ -118,6 +118,8 @@
     <!-- Miscellaneous -->
     <string name="error_allow_external_apps_ungranted">%1$s requires `allow-external-apps`
         property to be set to `true` in `%2$s` file.</string>
+    <string name="error_termux_service_start_failed_general">Failed to start TermuxService. Check logcat for exception message.</string>
+    <string name="error_termux_service_start_failed_bg">Failed to start TermuxService while app is in background due to android bg restrictions.</string>