Add LOCAL_LOGTAGS_FILES that prebuilts can use

Soong modules can define logtags files that need to be combined into
/system/etc/event-log-tags, so add a new LOCAL_LOGTAGS_FILES variable
that can be used to specify *.logtags file outside of LOCAL_SRC_FILES.

Bug: 28989759
Change-Id: I53c5d396dfb7c6006806758f351eb5cdde90fe74