To start using Swanalekha you don't need to learn any key mapping. Start with the common manglish you are familiar with.
Swanalekha is free and opensource. It works offline and does not collect any of your data.
With the SMC's Indic keyboard, the same input method is available in your android mobile. You don't need to learn another input method for mobile.
Type Malayalam with English letters: amma → അമ്മ. If the last letter is not correct, press Tab one or more times to see other letters: ka → ക, ഗ, ഖ, ഘ.
Keyboard shortcut: Ctrl+Shift+M