Add target_sdk_version for SoftKeyboard

CL [1] instroduced min_sdk_version due to createDisplayContext
needed sdk version 19+.

If we are using pre-release SDK version but the device didn't match
this platform and won't set target_sdk_version, it will result in
INSTALL_FAILED_OLDER_SDK error during install apk.

To make apk installable in older sdk devices & without hitting
pre-release SDK version case, specify target_sdk_version value to keep
its comparability.

[1]: Ic592a1d2fb2da149220c8b503b522b3e864bcc77

Fix: 137200759
Bug: 126930163
Test: manual as below test steps:
     1. Check out the latest master.
     2. source build/envsetup.sh
     3. lunch aosp_walleye-userdebug
     4. make -j SoftKeyboard (or tapas SoftKeyboard)
     5. Using "adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk"
        to install app in older platform rom build (i.e. android P or N)
     6. Make sure .apk file can installed without INSTALL_FAILED_OLDER_SDK error.

Change-Id: I5da196b5a1b257ec6d5b9e6f447a4734928f4580
1 file changed