Handle BACK shortcuts in PWM by injection

Current BACK shortcuts are handled in InputDispatcher and
Meta+`, Meta+Backspace, Meta+Left are converted to KEY_BACK
in InputDispatcher instead of being handled in PWM since BACK is
processed on app side instead of System fully handling it.

This blocks us from handling shortcuts like Meta+Ctrl+Left since
by the time keys reach PWM it is converted to Key_Back+Left and
doesn't get processed. Instead we wil handle everything in PWM now
and try to get similar behavior by injecting KEY_BACK whenever
Back shortcuts are pressed.

Bug: 290473497
Test: atest ShortcutLoggingTests
Test: atest BackShortcutTests
Change-Id: Idc60b303abb9ebfd0142e7686aabc4a39e529c19