Fix missing textservices dump in 'adb bugreport'

This CL fixes a regression that was introduced when we enabled
multiple users to use spell checker APIs at the same time [1].

It turns out that 'adb bugreport' invokes
  adb shell dumpsys textservices
with '-a' option.

With this CL '-a' option is no longer treated as an error in
TextServicesManagerService#dump().

 [1]: I06c27ef834203a21cc445dc126602c799384527b
      06a26240

Bug: 63041121
Fix: 72462172
Test: adb shell dumpsys textservices -a
Test: Manually verified that 'adb bugreport report.zip' contains valid
      result for TextServicesManagerService.
Change-Id: I0b40845f542b1db816303dabe41083fc76f8aeee